a). Field of the Invention
The invention is directed to a strip-shaped, elastically flexible measurement strip for length measuring devices or angle measuring devices which is provided with a graduated measurement scale.
b). Description of the Related Art
Such measurement strips are used industrially for length measurements and angle measurements, e.g., for machine tools or the like, wherein a measurement strip is fastened in a suitable manner, for example, at the machine bed of the machine tool or at its clamping table, and its scale is sensed by a measurement head which is suitably connected with the other part. In order to fasten the measurement strips, the measurement strip is either glued directly to its associated receiving base by means of a suitable glue or is fastened, via a measurement scale body in which the measurement strip is held, e.g., also by means of gluing, to this measurement scale body.
When the measurement strip is glued on, mechanically induced internal stresses inevitably build up therein, either because of the required contact pressure forces when gluing on, since there can never be contact pressure without longitudinal forces, or because of internal stresses resulting from the hardening or ageing of the glue, or because of thermal stresses due to different thermal expansion coefficients of the measurement strip and measurement scale carrier or receiving base. Because of this, substantial errors can result when measuring which considerably impair the accuracy of such a measuring system. A further possible source of error in measurement strips is the occurrence of bending in the measurement strip because the surface of the measurement strip which generally carries the measurement scale changes in length when the measurement strip bends. Local tensile stresses and compressive stresses, which also cause longitudinal errors, occur in the measurement strip depending in each case on the local position in relation to the neutral grain or fiber. This longitudinal error increases as the distance increases from the neutral fiber and therefore as the thickness of the measurement strip increases so that, to minimize this error, it is desirable to use the thinnest possible measurement strips. However, with decreasing thickness of the measurement strip the inherent strength or inherent stability of the measurement strip drops sharply, as a result of which minor mechanical stresses such as inevitably occur in the gluing process lead to the occurrence of noticeable errors, for which reason the use of thicker and accordingly more inherently stable measurement strips is desirable from this point of view.
An article in "F & M Feinwerktechnik und Me.beta.technik", Vol. 80, No. 7, pages 333ff, describes a measurement strip in which a groove is machined into a stainless steel measurement scale body to receive a thin steel strip with the measurement scale which is cemented therein. For the purpose of reinforcing the cement connection, the steel strip is connected laterally via welds with the associated carrier body. In this solution, although the carrier body and measurement strip are produced separately and the measurement strip can accordingly be produced with the desired length in an economical manner, the attainable accuracy of the measuring system is limited because the relatively low longitudinal stiffness of the thin steel strip, the unevenness in the strip, the inhomogeneity of the glue layer, and the scale errors caused by the bending of the scale body lead to errors which can no longer be overlooked.
A measurement device in which a scale is fastened in a flat manner on a metallic carrier by means of a silicone rubber glue layer is known from DE-PS 25 05 587. In this type of elastic fastening of the measurement scale on a stable base body, there are still extensive constraining forces which cannot be ignored and which can result in turn in deformations of the measurement scale, because the elastic adhesive forces of the glue layer can still be unacceptably large especially when the scales are relatively thin.
In another type of measurement system described, for example, in PCT-WO19/02919, a self-adhesive steel-strip measurement scale is aligned via an applicator to the operating sequence of the slide and is glued onto the machine bed or onto a base body. This known measurement system is very complicated to assemble and the attainable accuracy of the measurement system is only relatively limited, since the inherent stability of the measurement strip is rather low and even very slight mechanical stresses or differences in tension lead to corresponding errors. The linearity error of such glued measurement strips amounts to as much as approximately 40 .mu.m/m of the measurement strip. The attainable accuracy is accordingly sharply restricted and the scale errors must be corrected electronically by calibrating measurements, e.g., by means of a laser interferometer, which additionally complicates measurement and requires an electronic correction device.